Velocity of arc in medium

[2CRUDE]

gents, i have been debating a question with a collegue. he figures that a higher PD across 2 conductors of the same material, a set gap with a uniform medium will arc at a faster velocity than the same set-up with a lower PD. both PD's are more than capable of arcing the prescribed distance.
my thought was that as long as the PD is greater than the dielectric strength of the medium, the velocity will be the same.
is there any equation i can use to prove this?
go easy on the lost MechE.
tahnks
2C

 

[scottf]

PD? Is that potential difference?

I wouldn't think that the speed of the traveling arc is affected by potential at all. An arc is an arc, in terms of speed.

 

[2CRUDE]

PD=potential diff, my bad.
that's what i tought. if the voltage is sufficient to ionize the medium, the velocity of the arc will be the constant, as the medium ionization potential is fixed??
thanks for the reply!
anyone else?

 

[jghrist]

I'm not sure what you mean by the speed of the arc. If you mean how long it takes to flash over, then I would expect a higher voltage difference to be faster on the average. This is because there is a probabilistic factor in insulation flashover. For instance, the critical impulse flashover voltage of an insulator is the voltage at which it will flash over 50% of the time. A voltage close to the dielectric strength may not flash over the insulation immediately.

 

[Compositepro]

How do you define "velocity of an arc"? Once you do that you will probably be able to answer the question (which sounds intriguing).

 

[2CRUDE]

the question is in regards to spark plugs in high compression engines. my friend and i were discusssing a company that claims that a higher voltage will cause the spark to 'jump the gap faster' than a normal, already sufficient, voltage that is used.
my thoughts were that whether the applied voltage is 10 times or 100 times the dielectric voltage, the rate of ionization would not be increased and no benefit would be noted unless the gap length was greater, causing a larger spark. the company claims that the spark will form faster, and i don't see how if the voltages currently used in these set-ups are much greater than the dielectric V.
sorry if i'm not presenting the info clearly enough, it's not my specialty.
thanks
2C

 

[Compositepro]

There might be some truth in what your friend says. Breakdown of gas to form an arc is a cascade failure that takes some time to intiate but I can't say that it would be significant compared toi other effects. The rate that the voltage increases at the points of the plug is probably a bigger factor. A higher coil voltage may produce a higher dV/dt. But you can also advance spark timing for the same effect.

 

[burnt2x]

I agree with Compositepro.
Let me give you a very similar example (you ask us to go easy on you, that is why):
A welder unit does its function by initiating arc (arc welding). On SMAW (I know you know), the operator scratches the electrode on the work and lifts the elctrode a bit at a distance which the arc will be maintained to do his job. As long as the gap is maintained, welding process continues and ultimately the welding job will be completed. However, a newer welding process (TIG) uses high frequency-high voltage initiator to produce the arc (electrodes not touching) needed for arc welding which is very much easier for the operator compared to stick contact and lifting of the electrode handle. That, I think will give you the idea behind arcs!

The biggest factor then will be the voltage of the electric source so that arcs can be formed faster between the fixed gap.
To make it short, the claim that faster formation of arcs between a fixed spark gap can be achieved with higher voltage has truth in it.